This invention relates in general to improvements to cards containing selective media used to determine the identity, quantity and antibiotic susceptibility of microorganisms.
There are presently existing processes and apparatus for determining the presence, identity and quantity of microorganisms and their susceptibility to antibiotics which involve placing the microorganisms into micro-quantities of culture media some of which will sustain specific microorganisms. The media are chemically organized so that the optical characteristics of each changes in a predetermined manner when the medium is sustaining the desired type or types of microorganisms.
A convenient way to perform these processes is through the use of a card or cassette constructed with a rigid body in the form of a plastic plate, which includes viewing chambers or wells which pass through the plate. Each of the wells is connected to a predetermined liquid specimen supply port by means of channels cut in one side of the plate. Each port includes a septum, constructed from suitable sealing compounds, which allows passage of a hollow needle when a liquid specimen is being introduced into the port.
The card is constructed by first forming the plastic plate, inserting the septum and covering one side of the plate with adhesive tape. Chosen selective media in liquid form are then placed in the proper wells and freeze dried. The opposite side of the plate is then covered with another layer of adhesive tape to form, in effect, a sealed container with at least one supply port, a plurality of wells and a piping system therebetween. Unfortunately, before the liquid media are dried, they tend to flow down the connecting grooves by means of capillary or other action and mix with the media of other chambers. This is undesirable because it can cause erroneous results when the card is used.
To introduce a specimen into the viewing wells, a vacuum is drawn through a hypodermic needle inserted through the septum at the filling port. Then atmospheric pressure is used to force the diluted liquid specimen into the card. This causes both the wells and the adjacent branches to be full of liquid containing microorganisms. When no means are provided to prevent it, the microorganisms, which are sustained by the media in the viewing wells, tend to migrate with the reconstituted media through the branches to other wells. This is undesirable because it also can cause erroneous results when the card is read. There has also been a tendancy for the sealing compound which forms the septum to become dislodged during the card filling operation. This destroys the usefulness of the card.
Another problem results from the fact that some of the metabolic processes of the microbes produce gases which form bubbles in the viewing wells. Overflow chambers are normally provided adjacent the wells to provide for some bubbles such as those that result from the filling process, however, the metabolic bubbles tend to form in random fashion in the wells so that they interfere with the optical reading thereof.